A conventional electrical power generating system (EPGS) for an aircraft, in one known form, comprises an integrated drive generator including a constant speed drive and a generator. The integrated drive generator receives mechanical power at varying speed from an aircraft engine and delivers electrical power at constant frequency. The constant speed drive includes a speed control assembly and receives mechanical input power at varying speed from the aircraft engine and delivers power from its output shaft at constant speed. The generator comprises a salient pole machine with a rotating field which is excited through an exciter powered by a permanent magnet generator (PMG) through a voltage regulator. Such conventional systems use a generator control unit (GCU) to provide voltage regulation and speed regulation. Specifically, a voltage regulator provides excitation power to an exciter at levels which provide constant system voltage at the point of regulation. A speed control controls trimming of a servo valve to maintain generator speed, and thus frequency, to be constant.
Prior generator control units used either analog or digital circuits, with the choice being based on factors such as weight, size, cost and complexity of control logic. In analog systems both integrated circuits and discrete components are used and some signals are converted to digital form. However, signals are combined and perform their required functions using analog type control. Such system products incorporate standard, off-the-shelf components. Implementing a system which has the complexity of a generator control unit with standard product technology requires the use of many hundreds of electronic devices even for a relatively simple application, such as for a single channel EPGS. Each device adds additional weight to the product, including indirect weight in the form of additional circuit board area and housing needed to support the inclusion of each device. Since commercial and military aircraft are the intended end use of such products, it is desirable to minimize weight.
Further, analog circuits tend to be environmentally sensitive. For example, parameter drift results owing to changes in temperature and humidity, as well as age of the devices. Further, with analog technology the control cannot be easily changed. Instead, circuit components must be modified resulting in custom design for each different application.
In digital control systems, conversely, all signals are converted to digital form and certain control and protection functions are controlled by a microprocessor. As such, the control system is inherently more flexible in implementing different control schemes. In a digital control system the control unit contains a microprocessor and associated software and continuously and sequentially checks for proper system conditions and for control commands, and performs the automatic or command and control functions. However, the actual flexibility available with such a digital system is limited due to limitations in processing time available in the microprocessor for performing both control and protection functions. In fact, known GCU systems employ an analog control for implementing the voltage regulator functions. As a result, it is necessary to provide circuit components associated with voltage regulation.
Additional problems result in the design of generator control units. In each application it is necessary to develop a cost effective, lightweight solution. Therefore, the designer must start from "scratch" in designing a generator control unit for each new application. This results in each generator control unit being custom made and therefore more expensive.
A typical analog voltage regulator circuit includes a current limit circuit which compares peak sensed current to peak allowable phase current. When the sensed peak current exceeds a preselected reference value of, for example, 2.0 per unit current, then the control linearly reduces the voltage reference of the main voltage control loop up to a maximum peak current of, for example, 3.0 per unit, at which time the voltage reference is effectively reduced to zero. If current limit requirements change, however, these limit values can only be modified by altering circuit components. Also, owing to the strict linear relationship between current and voltage, the system may not react quickly under certain circumstances to limit current.
The present invention is directed to overcoming one or more of the problems discussed above.